#include <stdio.h>
int main()
{
    int x;
    double sum = 0;
    int cnt = 0;
    scanf("%d", &x);
    while ( x!= -1 ) {
        sum += x;
        cnt ++;
        scanf("%d", &x);
    }
    if ( cnt >0 ) {
        printf("%f\n", sum/cnt);
    }

    return 0;
}